Because questions 16-21 tend to contain question-types that most people find more difficult, begin with questions 1-15. This way, you'll start off on a roll and boost your confidence. Sounds reasonable enough, right? Here's where it gets wacky, though. After finishing question 15, jump to the end of the section and work your way backwards to eventually hit the toughest question. This technique isn't for everybody, but try it out on a couple of PrepTests and tell me how it works for you.